Bug 34256: Fix regression from 34092

Bug 34256: Fix regression from 34092

It appears search_fields variable got lost somewhere in my own rebases.
search_fields variable was not being considered as it should, in the function.
I think this patch fixes it.

This showed in the borrowernumer no longer being searchable:

* Use search filters in patron search
* Set 'Search field' to borrowernumber
* Search for 19 (Henry) or 41 (koha) in sample data
* No results :(
* Apply patch
* Search leads to the patron record again

Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>